A complete basis set model chemistry. II. Open-shell systems and the total energies of the first-row atoms

The major source of error in most ab initio calculations of molecular energies is the truncation of the one-electron basis set. An open-shell complete basis set (CBS) model chemistry, based on the unrestricted Hartree–Fock (UHF) zero-order wave function, is defined to include corrections for basis set truncation errors. The total correlation energy for the first-row atoms is calculated using the unrestricted Mo/ller–Plesset perturbation theory, the quadratic configuration interaction (QCI) method, and the CBS extrapolation. The correlation energies of the atoms He, Li, Be, B, C, N, O, F, and Ne, calculated using atomic pair natural orbital (APNO) basis sets, vary from 85.1% to 95.5% of the experimental correlation energies. However, extrapolation using the asymptotic convergence of the pair natural orbital expansions retrieves from 99.3% to 100.6% of the experimental correlation energies for these atoms. The total extrapolated energies (ESCF+Ecorrelation) are then in agreement with experiment to within ±0.0012 hartree (root-mean-square deviation) and represent the most accurate total energy calculations yet reported for the first-row atoms.